River Features

I

Introduction

River Features, elements of the landscape produced by fluvial processes—that is, the action of running water as it flows through the channels forming the drainage network of a river basin, eroding, transporting, and depositing sediment. The term “river” is used for convenience throughout this article, but it is important to remember that fluvial processes affect all drainage channels regardless of size—from the smallest streamlet to the world’s mightiest rivers. In fact, although the effects upon the landscape of large rivers tend to be the most dramatic, it is from detailed studies of small streams that much of our knowledge of fluvial processes has been gained.

All rivers consist of a flow both of water and of sediment—materials derived from rock and organic matter that can vary in size from fine clay particles to huge boulders. The features produced by a particular river thus depend not only on the characteristics of the water flow, notably its volume (discharge), distribution over time, and energy, but also on the amount and size of the sediment load (sediment discharge). The third contributory factor is the geology of the basin, which helps condition the type and amount of sediment, and also affects the amount of work a river has to do and the features that result, because some rocks are more resistant than others.

II

River Processes

The primary processes responsible for the formation and evolution of rivers and their features are erosion, and sediment transport and deposition. Rivers are able to do work on the landscape because the energy stored in the water, or potential energy, is translated as it flows downhill owing to gravity into the kinetic energy used for erosion, transport, and deposition. The amount of potential energy available to a river is proportional to its initial height above sea level. In order to minimize the loss of potential energy to thermal energy, or heat, as a result of friction, and thus maximize available kinetic energy, the river follows the path of least resistance downhill. Even so, it is estimated that 95 per cent of a river’s potential energy is used to overcome friction, which occurs mainly along the channel boundaries (the bed and banks), although the internal friction of the water and air resistance on the surface are also important.

There are two main patterns of flow: laminar and turbulent. Laminar flow is an even, horizontal movement, in which the water flows in clearly defined layers over sediment on the bed without moving it. Laminar flow is considered to exist in rivers more in theory than in reality and is usually discounted. Turbulent flow, the dominant pattern, consists of a series of erratic vertical and horizontal eddies moving in a downstream direction. Turbulence varies in direct relation to the velocity of flow, which in turn is linked to the amount of kinetic energy available. The greater the kinetic energy, the greater the velocity (and vice versa), and the greater the turbulence of flow.

A

Erosion

Erosion is the means by which a river deepens, widens, and lengthens its channel. There are several main erosional processes.

Hydraulic action occurs because the energy of the flowing water hitting the boundaries of the river channel, especially the banks, is sufficient to prise away fragments of the bedrock. This hydraulic shearing force is caused by the fact that water is forced into cracks in the bedrock. The air in the cracks is compressed and, as a result, pressure increases. Over time this weakens the rock and fragments break away. An extreme form of hydraulic action, associated with waterfalls and rapids, is cavitation. It is caused by air bubbles collapsing. The resulting shock waves hit and weaken the channel boundaries, and may eventually cause the banks to collapse. Hydraulic action is measured in terms of shear force per unit area, which is termed the boundary shear stress. As well as eroding fragments of intact bedrock, the river may also erode loose particles—variously termed scree, talus, or colluvium—that have accumulated at the base of slopes after being detached from the bedrock by weathering processes such as freeze-thaw, the growth of salt crystals, or the actions of plants and animals. This process is termed sediment entrainment.

The efficacy of flowing water in erosion is greatly assisted by the battering effect against the bedrock of the channel of sediment already in motion, a process called corrasion. Corrasion is responsible for much of the downcutting that creates and deepens the channel, and is most effective in times of flood. A particular form, potholing, occurs when pebbles or other sediment are trapped in hollows in the bed and are swirled around by turbulent eddies, scouring out and deepening the depression. It is not only the channel that is worn away by this process; the sediment load is itself abraded by collisions between individual particles and between particles and the channel boundaries. This process, sometimes called attrition, reduces the size of transported particles with distance downstream and also gives them the rounded shapes typical of river cobbles and pebbles. Finally, water is also a strong solvent. Many rocks can be eroded by being dissolved by water, a process known as corrosion, or solution. Limestone and chalk are particularly susceptible to corrosion, but many chemical compounds are soluble, particularly in their weathered state, so a wide range of rocks may be vulnerable.

B

Transport

Eroded material is carried downstream by the river, together with sediment that is washed into the channel by overland flow, or surface run-off—the flow across the land surface of water that accumulates when rainfall exceeds the capacity of the soil to absorb it—as the total sediment load. The total load can be subdivided on the basis of its origin, into three categories. Dissolved load is the sediment load derived from corrosion and chemical weathering. Wash load is the sediment washed into the channel by overland flow. It is much finer than that found on the channel bed. Third, bed-material load comprises sediment eroded from the channel boundaries and is similar in size to the bed material.

The mechanics and speed of movement of sediment making up the total load vary depending on the size of the particles. The movement of dissolved-load sediment corresponds to that of the water it is dissolved in. The wash load and the finer particles of the bed-material load are mixed with the water by turbulent eddies caused by shearing between the water and the channel boundaries. These eddies carry silt- and fine-sand-size particles above the bed of the river for long distances as suspended load. However, larger particles—coarse sands, gravels, cobbles, and boulders—are too heavy to be lifted by turbulence, and they slide, roll, and bounce (or saltate) along the bottom as bed load. The largest cobbles and boulders may be moved only during times of flood. The proportions of sediment moved by different mechanisms vary enormously between rivers, and can also vary within the same river at different times, such as during a flood. However, as a general rule, the suspended load constitutes between 70 per cent and 85 per cent of the total sediment load.

There is a strong relationship between the velocity of flow in the river, the boundary shear stress, and the size of particles eroded, transported, or deposited. In the early 1930s the Swedish scientist Filip Hjulström carried out experiments to define the velocities necessary to initiate the movement (or erosion), transport, and deposition of sediment of different sizes. Hjulström presented his results in 1935 as a graph showing the relationship between velocity (y-axis) and sediment diameter (x-axis) in the form of two curves: one plotting the critical erosion (or entrainment) velocity—that is, the velocity at which particles of a given size can be eroded from a bed of loose sediment and movement thus initiated; the second plotting the critical fall, or settling, velocity at which deposition is initiated. Between them transport will occur; Hjulström found that, once in motion, particles do not require such high velocities to continue being moved. The critical erosion velocity is lowest for sand-size particles. Higher velocities are necessary to entrain both finer and coarser sediment particles. Finer particles, such as silt and clay, require a greater critical velocity to get them moving because of their cohesive nature; in other words, they stick together better. For coarser sediments such as gravel, pebbles, and cobbles the higher critical velocity is purely a result of their greater weight.

The maximum size of particles transportable by a river is called its competence. A river’s competence is related to both boundary shear stress and velocity. Maximum particle size increases in a direct linear fashion with increases in boundary shear stress. However, the relationship between increases in velocity and increases in particle size is governed by the so-called sixth-power law. According to this, whatever factor the velocity increases by, then the mass of the largest particle is that factor multiplied to the power six. For example, if the river’s velocity increases by a factor of four then the mass of the largest particle that could be moved would increase by 4⁶, or 4,096 times. These relationships can be used to define the competence of a river for a given boundary shear stress or critical velocity.